Biogas quality analysis. Get the sampling process right

The need for sampling of biogas quality at different stages of the AD process is a given. It is a vital part of plant management and ensuring that output is financially viable. But have you thought about the way in which sampling takes place?

In the last of three articles on biogas monitoringChris Dakin, who has been at the heart of Gas Datas innovative gas analysis technology for over 20 years, provides some insights into a better sampling process.

Fixed site gas analyserIn my last piece about flow measurement I highlighted how important it was to take a very rigorous approach to the measurement process. The choice of instrument technology and the detail of the analysis could make major differences to the results and, therefore, to the management actions taken as a result.

The same rigorous approach should be applied to the gas quality analysis too by measuring the gas concentrations at each stage and under known conditions of pressure and water content. Almost all systems currently on the market will analyse methane and carbon dioxide using infra- red adsorption analysis combined with electrochemical based sensors for oxygen, hydrogen sulphide and occasionally ammonia. Some systems exist that are in pipe or in vessel which have the great advantage of avoiding the necessity to pump small samples of the biogas to a central analyser. However, the hostile environment inside the biogas process makes this method extremely difficult so this type of analyser is rare and are yet to achieve an acceptable track record in the industry.

Extracted sample systems are very much the norm. Small bore (3mm to 8mm) pipes are used to carry biogas samples on demand to an analyser system. A single analyser can be time shared between as many sample points as necessary making this a highly cost effective method in terms of capital outlay and running costs. Sampling systems are almost always flexible in the way that they are programmed giving options for sample rates and sequences. They can be optimised from one to ten or more sample points scanning each in a programmable order typically completing a single cycle of all points in fifteen to sixty minutes depending on the size of the plant.

A major advantage of using a single analyser fed sequentially from the different locations on the plant is that any error due to analyser calibration drift will be equal across all measurements so when subtracting one reading from another to make comparisons between stages the error is cancelled. This means small differences between them can be more easily resolved. For a system using multiple analysers this advantage is lost. A further advantage of sequential sampling systems is for one of the sample points to be connected to a cylinder of accurate calibration gas. This gas can then be used each cycle or each day to verify the calibration status of the analyser automatically.

Measurement of gas quality and quantity not only measures the efficiency of feedstock consumption but also allows energy production efficiency to be calculated. Only by knowing the exact quantity of methane being consumed by a generator can its conversion efficiency be calculated accurately.

fixedBiogas will almost always contain sulphur in the form of hydrogen sulphide gas. This gas is odorous, poisonous and is the source of metal corrosion when present in both its raw form and when combusted in the engine to form sulphur dioxide (SO2). The need to monitor this gas and control its generation is most important. All engine manufacturers will provide guide lines that state the additional maintenance requirements that must be put in place for any given level of hydrogen sulphide in the gas. Without accurate knowledge of the levels then there is a real risk maintenance will either be carried out unnecessarily or even worse be left undone. Both outcomes can be very expensive.

The addition of instrumentation to a biogas plant does require careful planning if accurate results are to be obtained but it need not be onerous. Practical and theoretical limitations of the instruments must be taken into account such as temperature and pressure ranges, expected gas concentrations ranges (particularly that of hydrogen sulphide), lengths of sample pipes etc.

In addition appropriatestandards and work practices should not be ignored. In the UK the Health and Safety Executive (HSE) publish documentation called the Dangerous Substances and Explosive Atmosphere Regulations (DSEAR) which provides a framework for the structure of plant and in its turn refers to the need for suitable equipment ratings such as ATEX.

Fortunately a number of educational establishments, such as Reaseheath College in Nantwich, UK, provide courses for the necessary skills of running biogas plants efficiently and safely. This is a sign that the monitoring of biogas is becoming an integral part of biogas plant operation.

In a sense these articles have only scratched the surface of what is an emerging and evolving management activity. If you want any advice on the monitoring, sampling, measurement or analysis of biogas then call us today on +44(0) 247 630 3311 or via email:enquiries@gasdata.co.uk.

Part one – Producing Biogas from AD. Don’t fly blindfolded.

Part two –Biogas Production. Measuring output to manage input.